The long-term aim of this proposal is directed to an understanding of the structural organization of a gene in a higher eucaryote, and the relationship of that organization to the time and tissue specific control during development of the expression of that gene. The rosy locus of Drosophila melanogaster, which codes for enzyme, xanthine dehydrogenase continues to be our major model experimental system. However, we have expanded our effort to consider the genes immediately proximal to rosy, namely S12 and the mes complex, as well as the scalloped locus on the X-chromosome. The S12 and sd studies are designed to question problems of position effects on gene expression, while the mes complex represents a large gene with a broad array of developmental efforts. A second aim is concerned with the mechanism of recombination in a higher eukaryote. The methods to be utilized continue those genetic and molecular tools used in our prior work. Recently, we have started to utilize gradient denautrationgel mapping to localize small DNA lesions, and to sequence important regions. This will continue. Additionally, we hope to develop a system for homologuous transformation, and to produce nonosene suppressor mutations.